What is the cause and how can I fix it when I have pain in the left side of the rib cage under the breast?

Dietary, gas, spleen. Kimberly, there are air pockets that can move if you touch your toes and move about, then lie on the side of the pain on the ground. Repeat this process if it helps a little... If you are constipated eat metamucil and yoghurt (organic fat free, plain). Drink a glass of water afterwards, wait and repeat the above. If you have family with sickle cell blood get yourself checked.
Usually on its own. This is usually a condition called "spleenic flexure syndrome". There is a "bend" in the large intestine in the left upper side and at times gas or stool can become "trapped" between the spleen and diaphragm. Lie down own your left side and gently massage the area. This is a common and harmless condition.